Conventional long haul communication systems comprise both optical and electronic components. For example, repeaters detect light photoelectrically, amplify the resulting current electronically and then use the amplified current to drive a semiconductor laser that reconverts the electrical signal back into an optical signal. The optical signal is then injected into an optical fiber to the next repeater in the system where the conversion from optical to electrical and back again to optical is repeated again.
In an all-optical transmission system, light once generated will be transmitted optically, received by optical detection and, more importantly, amplified optically such that there is no intermediate conversion from optical to electrical and then back to an optical form. Optical amplifiers afford direct optical amplification of an optical signal, which results in the elimination of the electronic processing. Accordingly, optical amplifiers will enable optical communication systems to have repeaters which have higher bandwidths, are physically smaller, simpler in design, more efficient to operate and more economically to produce.
Furthermore, with the performance of optical amplifiers relatively unaffected by changes in data bit rate or by the presence of additional channels at separate wavelengths, optical amplifiers will become key components in lightwave transmission and switching systems. Unfortunately, although optical amplifiers are integrable with other opto-electronic devices in photonic integrated circuits (PICs), it has been problematic to control the output power because the gain of an optical amplifier can be affected by both environmental effects, such as changes in source wavelength and polarization, as well as temperature variations and amplifier degradation. Accordingly, integrating a monitoring detector with the optical amplifier to monitor the output power therefrom seems to be a reasonable solution, other than utilizing bulk detectors and couplers, which are prohibitively lossy and expensive.
Unlike a semiconductor laser wherein a detector may be positioned on the back facet, no facet of the optical amplifier is available because both front and back facets are employed for ingressing and egressing optical radiation. As such, the only viable alternative is to employ an integrated branching waveguide, such as a Y-junction waveguide, to tap a fraction of the output power to monitor the amplifier. Because the injected optical radiation into the optical amplifier can be arbitrarily polarized, the power splitting ratio of the branching waveguide should be polarization invariant in order for the photogenerated current of the integrated detector to be used in a feedback configuration. Disadvantageously, optical amplifiers require more than 40 dB of optical isolation in order to suppress ripples in the gain spectrum resulting from residual Fabry-Perot resonances. Alternatively stated, the back-reflectivity of the Y-junction waveguide should be substantially smaller than the residual reflectivities of the anti-reflective coatings on the end facets of the optical amplifier.
Due to fabrication limitations, practical Y-junction waveguides have truncated wedge tips. See, for example, Sasaki et al., Electronics Letters, Vol. 17, No. 3, pp. 136-8 (1989). That is, a blunted Y-junction tip, which results in a substantial amount of optical back-reflection to the optical amplifier which has, because of deleterious effects to the amplifier, prohibited the monolithic integration of a coupled optical amplifier and monitoring photodetector via an optical branching waveguide.